Discriminated unions used

Optional boolean flags for mutually exclusive states — isLoading, isError, isSuccess on the same object — allow impossible combinations that the type system cannot prevent: { isLoading: true, isSuccess: true } is a valid TypeScript value, and code that doesn't defensively check every combination will produce undefined behavior. ISO 25010:2011 §6.5.4 classifies this as a reliability defect. Discriminated unions encode the constraint that only one state can be active at a time directly in the type, eliminating an entire class of conditional-logic bugs.

Replace multi-flag state types with a discriminated union keyed on a status or type field. TypeScript then exhaustively checks all cases at switch sites:

// Bad: flags allow impossible combinations
type FetchState = {
  isLoading?: boolean
  isError?: boolean
  data?: User
  error?: string
}

// Good: only one state possible at a time
type FetchState =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: User }
  | { status: 'error'; error: string }

switch (state.status) {
  case 'idle': return <IdleView />
  case 'loading': return <Spinner />
  case 'success': return <UserView user={state.data} />
  case 'error': return <ErrorView message={state.error} />
}